1. Field of the Invention
This invention relates to a sound quality adjustment circuit. More particularly, it relates to a sound quality adjustment for adjusting frequency characteristics of an audio signal range.
2. Description of the Related Art
For adjusting the sound quality of the audio signal depending on the conditions of the sound source or the playback sound field or on the hearing power or the preference of the listener, a tone control circuit for continuously changing the gain of a specified frequency range or a graphic equalizer circuit for splitting the frequency spectrum into plural bands and for changing the gain in each of the split bands is used extensively.
FIG. 1 shows the manner of sound quality adjustment by the tone control circuit. In an instance shown in FIG. 1, the center frequency of the band in which to adjust the sound quality of the audio signal is fixed at 1 kHz. The gain can be changed continuously in a lower side within a band of the turnover frequency of f.sub.L1 to f.sub.L2 and in a higher side within a band of the turnover frequency f.sub.H1 to f.sub.H2. In this manner, the tone control circuit adjusts the sound quality in accordance with pre-set frequency characteristics, while it cannot augment or attenuate the gain within each split band.
Conversely, the graphic equalizer circuit splits the audio signal into plural frequency bands to augment or attenuate the gain in each of the split frequency bands. FIG. 2 schematically shows the manner of sound quality adjustment by this graphic equalizer circuit. In the present instance, the frequency spectrum is split into five bands.
However, the center frequencies f1, f2, f3, f4 and f5 of the frequency bands of the graphic equalizer circuit are usually fixed at pre-set values, and cannot be changed freely. Thus, there are occasions wherein the signals of desired frequencies cannot be augmented or attenuated.
In FIGS. 1 and 2, the response on the ordinate represents the intensity ratio (gain) of an output signal to an input signal. The response portion above 0 dB and that below 0 dB indicate that the signal is augmented and attenuated, respectively.
FIG. 3 shows an illustrative structure of such a graphic equalizer circuit.
The graphic equalizer circuit includes an input terminal 111, to which an input signal Vi is applied, an operational amplifier 112, having an non-inverting terminal and an inverting terminal and an output terminal 113 for outputting an output signal Vo. To the inverting input terminal of the operational amplifier 112 is entered the output signal Vo via a negative feedback resistor Ro.
The band-pass filter 31A has a center frequency .omega..sub.0 and a transfer function H(s). A pair of voltage-current converters 32A, 33A voltage-current convert an output signal of the band-pass filter 31A. The voltage-current converters 32A, 33A are constituted by current mirror circuits as later explained. The values of conversion conductance of the first and second voltage-current converters 32A, 33A are (1-a)gm and (1+a)gm, respectively, where -1&lt;a&lt;1.
If the graphic equalizer circuit is constructed for splitting the frequency into N portions, N sets each made up of the band-pass filter 31A, first voltage-current converter 32A and the second voltage-current converter 33A are arranged in parallel as indicated by dotted lines in FIG. 3. The signals of the first to Nth bands, having the gain changed by N band-pass filters 31A, 31B, 31C, . . . , 31N, taking charge of the split N bands, are summed by the operational amplifier 2 and outputted as an output signal V0 at an output terminal 3.
The frequency characteristics shown in FIG. 2 correspond to those of the output signal Vo obtained by splitting an input signal into five frequency bands, changing the gain values of the five band signals by band-pass filters 31A, 31B, 31C, . . . 31H taking charge of the spit five bands, summing the respective spilt signals by the operational amplifier 2 and outputting the sum signal at an output terminal 3.
By employing the graphic equalizer circuit, it becomes possible to split the audio signal into plural frequency bands and to augment or attenuate the gain optionally in each of the split frequency bands. However, since the center frequencies of the split bands are fixed, there are occasions wherein signals of the desired frequency cannot be augmented or attenuated with a smaller number of divisions of the frequency spectrum.
Of course, if the number of divisions of the frequency spectrum is increased and the gain is changed in each of the split frequency bands, it becomes possible to change the gain only of the signal of the desired frequency optionally. However, this method is not desirable for application to a practical circuit required to be small in size and inexpensive because of the increased circuit scale and raised cost.